1. Field of the Invention
The present invention relates to an image display apparatus and, more particularly, to a head- or face-mounted image display apparatus which is retained on a user's head or face to project an image into an observer's eyeball.
2. Description of the Related Art
In recent years, head- or face-mounted display apparatuses have been developed for the purpose of enabling the user to enjoy a wide-screen image personally.
In such circumstances, Japanese Patent Application Unexamined Publication (KOKAI) Nos. 7-333551 (1995) and 8-234137 (1996) propose an ocular optical system which leads to a position of an observer's eyeball an image formed by an image display device comprising a liquid crystal display (hereinafter called "LCD"). This ocular optical system is constructed by a decentered optical system made of a medium which is surrounded by three optical surfaces and has a refractive index larger than 1, wherein light rays emitted from the LCD enter the decentered optical system from a third surface, are totally reflected on a first surface inside, are reflected on a second surface forming a concave mirror, and exits through the first surface to the outside of the decentered optical system, thereby leading an image of the image display device to the observer's eyeball without forming an intermediate image.
Japanese Patent Application Unexamined Publication (KOKAI) Nos. 7-333551 (1995) and 8-234137 (1996) each employ a transmission-type LCD as the LCD constructing the image display apparatus. On the other hand, Japanese Patent Application Unexamined Publication (KOKAI) No.7-72446 (1995) proposes a face-mounted image display apparatus in which a reflection-type LCD is used as an image display device. FIG. 42 of the present application shows an optical system of the image display apparatus, in which light rays from a lamp light source 55 are made parallel rays by a collimator optical system 56, and part of the light rays (s-polarized light) are reflected by a polarization beam splitter 57 to illuminate a reflection-type LCD 58 from its front side. An image displayed on the reflection-type LCD 58 is projected by an optical system 59 onto a screen 52, and the projection image is observed by an observer through an ocular optical system 53.
In addition to the reflection-type LCD as the reflection-type image display device, an image display device called DMD (digital micro device) has also been proposed. This device has a structure shown in FIGS. 43(a) and 43(b). That is, FIG. 43(a) shows a plane view and FIG. 43(b) shows a structure of each element. In this structure, micro mirrors 60 corresponding to respective picture elements are arranged two-dimensionally. By tilting a mirror 60' at a designated position about its diagonal axis, light rays incident on the mirror 60' from a fixed direction are reflected in a direction different from the reflecting direction of the mirrors which are not tilted, thereby displaying a two-dimensional image. Each of the mirrors 60 is supported via hinges 63 at a pair of diagonally opposite corners by posts 62 set up on a substrate 61. Each of the mirrors 60 is rotatable about its diagonal axis between the hinges 63 by an electrostatic force generated by applying a voltage to one of a pair of electrodes 64 provided behind the mirrors 60 on the substrate 61 (IEEE Spectrum, Vol. 30, No. 11 pp. 27-31).
When a decentered optical system is employed as an optical system of an image display apparatus, there are the advantages that the whole apparatus can be made small and lightweight, keeping high optical performance (field angle, resolving power, etc.), and that the image display apparatus can display a bright image. Up to now, however, it has been intended to use only a transmission-type LCD as the image device for use with the decentered optical system. Therefore, each of the decentred optical systems disclosed in Japanese Patent Application Unexamined Publication (KOKAI) Nos. 7-333551 (1995) and 8-234137 (1996) has been constructed only as a telecentric optical system in which the position of an entrance pupil is set at an infinity point.
Further, the transmission-type LCD as an image display device has a lower vignetting factor of the picture element compared to the reflection-type LCD, and black matrices between the picture elements are conspicuously observed. Therefore, it is necessary to use a low pass filter (which cuts high spatial frequencies and passes low spatial frequencies) or the like so that the black matrices will not be conspicuously observed. On the other hand, in the reflection-type LCD, it is possible to make the vignetting factor of the picture elements higher, and the aforesaid problem is small. However, as disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. 7-72446 (1995), when a non-decentered optical system is used to observe an image, illumination must be made through an optical element such as a beam splitter. This is contrary to the requirement of miniaturization and weight reduction, and there remains a problem that a displayed image is dark.